Forging process

ABSTRACT

Process for manufacturing a forged article in which a preform is generated by filling a semi-rigid liner with powdered metal, providing the liner with a sealing element, and isostactically compressing the liner and its contents. The liner is then removed from the green preform, the preform is sintered, and the resulting sintered preform is forged.

.Iadd.This is a continuation of application Ser. No. 98,627 filed Nov.29, 1979, now abandoned, which is a reissue of U.S. Pat. No. 4,094,053..Iaddend.

BACKGROUND OF THE INVENTION

Forging is a process by which the shape and physical properties of metalcan be changed. The process involves placing the piece of metal(normally heated) between the halves of a die and forcing the die toclose by impact or pressure. The operation causes a controlled plasticdeformation of the metal into the cavities of the die. This flow ofmaterial results not only in a change in the shape of the metal, butalso improves the uniformity of the metal, improves its grain structure,and causes a shape-conforming grain flow. The resulting workpiece hasproperties which are superior to those generated by other methods, thusmaking forging essential where high performance workpieces are required.

One of the difficulties in the forging process is that, when a complexshape is to be manufactured, the workpiece which is fed to the finalforging die must be relatively close to the final desired shape. Thisworkpiece (which is shaped properly to be placed in the final forgingprocess) is called a "preform." The design and formation of this performcan be critical to the economics of the forging process, since thesefactors affect the useful life of the final forging dies and the powerrequirements of the press and the amount of waste of metal in the flash.

In the past, two general procedures were used to develop the preforms.Where maximum performance of the workpieces is required, a series offorging operations can be used. The standard mill stock may be exposedto several roller impressions, some bending and twisting impressions,gathering and conditioning impressions, and then blocking impressions.Each of these operations requires substantial expenditures forspecialized forming equipment and the cost of the operations can only bejustified in extreme situations. In workpieces in which the strengthrequirements are not as rigorous, the preform can be machined from themill stock using standard cutting methods. This process can involve agreat deal of wasted material and, its practicality is severely effectedby the spiraling increase in the cost of the various resources necessaryto carry it out.

It has been suggested that the shortcomings of the above processes couldbe eliminated in certain situations by the formation of forging preformsfrom powdered metal. When a powdered metal is forged, its porosity issubstantially reduced and its mechanical properties may be improved, sothat they are equal or close to those obtained in forged conventionalmetal. While the arguments in favor of such a process make a great dealof sense, the practical problems of carrying out an economic system(particularly where large or complex shapes are involved) have not beensatisfactorily solved. If powder metallurgical preforms are made in"hard" tooling, the presence of multi-level portions in the pressingdirection, the presence of side wall friction, and the presence of tallparts all pose severe problems for their manufacture. When the preformis made with "soft" tooling, as used in isopressing, the need fortime-consuming bag filling, supporting, bag stripping, and pressurizingand de-pressurizing cycles tend to reduce production rates tounacceptably low levels. These and other difficulties experienced withthe prior art processes have been obviated in a novel manner by thepresent invention.

It is, therefore, an outstanding object of the present invention toprovide an efficient and economically advantageous process for formingforged articles.

Another object of this invention is the provision of a process in whichthe preforms of the forging process are generated from powdered metal.

A further object of the present invention is the provision of a forgingprocess in which the preforms are formed by casting powdered metal insemi-rigid molds, isostatically compacting the mold and its contents,removing the mold from the compacted contents, and sintering the greencontents to generate the preform.

It is another object of the instant invention to provide a forgingprocess in which a powdered metal preform is developed in a method whichcan be adapted to automation, because the steps involve relativelysimple mechanical operations which can be carried out in a mannerconsistent with modern manufacturing techniques.

A still further object of the invention is the provision of a forgingprocess which results in forged articles which have mechanicalproperties equal or close to articles manufactured by conventionalmethods.

With these and other objects in view, as will be apparent to thoseskilled in the art, the invention resides in the combination of stepsset forth in the specification and covered by claims appended hereto.

SUMMARY OF THE INVENTION

This invention involves a process for manufacturing forged articles frompowdered metal preforms. The process involves preparing a semi-rigidliner of a shape suitable for a forging preform, filling the liner withpowdered metal, providing it with a sealing element, and thenisostatically compressing the liner and its contents. After that, theprocess involves removing the liner from the green preform, sinteringthe preform, and then forging it. After compaction, the liner is removedfrom the contents either before or after a softening heat step bydecomposing the liner in the sintering furnace.

BRIEF DESCRIPTION OF THE DRAWINGS

The character of the invention, however, may be best understood byreference to one of its structural forms, as illustrated by theaccompanying drawings, in which:

FIG. 1 is a perspective view of a forged article manufactured by aprocess embodying the principles of the present invention,

FIG. 2 is a diagrammatic representation of the isostatic compactionequipment that might be utilized in the process of the presentinvention,

FIG. 3 is a flow chart showing the generalized steps of the presentprocess,

FIG. 4 shows the detailed steps of one version of the present process,

FIG. 5 shows the detailed steps of another version of the presentprocess,

FIG. 6 shows the detailed steps of another version of the presentprocess,

FIG. 7 shows the detailed steps of another version of the presentprocess,

FIG. 8 shows the detailed steps of another version of the presentprocess, and

FIG. 9 shows the detailed steps of still another version of the presentprocess.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a workpiece 10 which has characteristics making itparticularly suitable for manufacture by the present forging process.The article is shown as a connecting rod for an internal combustionengine and, therefore, for well-known reasons, it must have thestrength-to-weight and strength-to-volume ratios which are now achievedby a forging process. The shape of the connecting rod is sufficientlyirregular that the preparation of a suitable preform for finish forgingwould require several complex, expensive, and wasteful operations as itis made by using conventional prior art methods. A suitable preform fromwhich to forge this article is relatively large (approximately 20 incheslong) and, as a result, is particularly unsuited to be made by prior artpowdered metal techniques. This is not only because it is too large tobe processed practically in standard "hard", powdered metal tools, butalso the unsintered preform would have to have substantial strength inorder to be handled without fracturing. It should be noted that, whilethe present process is particularly suited for this type of workpiece,it is also useful in forming less challenging articles.

Referring now to FIG. 3, which shows a general scheme of steps making upthe forging process of the present invention, the process, indicatedgenerally by the numeral 11, begins with the purchase of powder in Step12 and the manufacture of plastic liners or container in Step 13. Thepowdered metal is of the type well-known in the powdered metal formingart, and in the preferred embodiment is of the atomized alloy-steelpowdered metal designated as modified 46XX 46XX or 46FXX 46FXXcompositions. Although the details of the manufacturing process for theplastic liners will be described in detail below, the function is toproduce a thin-walled, inexpensive, disposable liner formed of amaterial such as expanded polystyrene and having an internal cavity of aprecisely desired shape. The desired shape will be a shape determined byreverse engineering, backward through the remaining steps in theprocess. The characteristics of the liner are that it be made of aninexpensive material, that it be capable of collapsing against thepowder under the influence of the external isostatic pressure, withoutsevere angular distortion. Some means may be provided to cause the linerto maintain its precise internal shape while being filled with thepowder. Such factors will be discussed in further detail hereinafter.

In Step 14, the liner is filled with the metal powder. It has also beenfound that vibrating or rotating the liner during or after the fillingStep 14 provides a more uniform and, therefore, more predictable powdermass.

In Step 15, the liner and its contents are isopressed, that is, exposedto isostatic pressure. While this isopressing can be accomplished by anumber of methods, in the preferred embodiment of this invention, it iscarried out by the so-called dry-bag method as shown in FIG. 2. In thismethod, the isopress 16 has a cavity 17 which is closable by a pressurecap 18. The cavity 17 is divided into an inner chamber 19 and an outerchamber 20 by a flexible rubber bag 21. The workpiece 10, consisting ofthe plastic liner filled with metal powder, is placed in the innerchamber 19 and the cavity 17 is sealed by cap 18. The outer chamber 20is then filled with high-pressure fluid which causes the rubber bag 21and liner to collapse against the contents of the liner, and exerts anisostatic, omni-directional pressure on all surfaces of the workpiece10. This isostatic pressure causes a compaction, consolidation, andstrengthening of the now "green" workpiece 10. The high pressure fluidis provided by a system including a reservoir 22, a pump 23, and anaccumulator 24, and an intensifier 25. The elements are piped togetherso that the pump feeds through the accumulator to an intensifier loopand thence into the outer chamber 20. Valves are provided to control thesupply of pressure and to release the pressure from the outer chamber 20at the end of the isopressing cycle. At the end of the isopressingcycle, the pressure is released from the outer chamber 20. As a result,the rubber bag 21 returns to its original condition because the linerprevents the rubber bag from interlocking with the powder during thepressing. However, the plastic liner will normally set to the new,compacted shape of its contents. This relationship of the variousflexible elements in the system minimizes the stresses imposed on the"green" workpiece and helps to prevent breakage.

After the workpiece 10 or preform has been removed from the isopress 16,it may be exposed to a preheat Step 26 which prepares it for sintering.This preheat step would be of the type normally used to prepare powderedmetal "green" workpieces for sintering. Preheating may be used to softenthe liner for removal or it may be used to decompose the liner.

Next the sintering Step 27 is carried out on the workpiece. The preformis then forged in Step 28, heat-treated in Step 29, tested in Step 30,machined, if necessary, to final form in Step 31, and then shipped inStep 32. The result of this process 11 is a workpiece which possessesthe desirable characteristics of a conventionally-forged article and,yet, avoids the substantial expense and wastage of forming a billet fromthe molten metal and then shaping it into the required preform.

Two areas of the process require further discussions; the techniquesused to form and fill the liners, and the technique used to remove theliner from the isopressed preform.

In the preferred embodiment, the liner would be formed and filled usingtechnique C shown figuratively in FIG. 6. Likewise, in the preferredembodiment, the liner would be removed using technique Z shown in FIG.9.

Referring now to FIG. 6, Step 46 involves the manufacture of the liner.Step 46 would normally be carried out at a remote location and interfaceonly batch-wise with the remainder of the process. The manufacture couldinvolve numerous well-known methods for generating a thin-walled, hollowliner of polymeric material. In Step 47 the liner is filled with thepowdered metal as previously discussed. The liner and its contents areplaced in the flexible bag 21 of the isopress 16 in Step 50 andisopressed in the normal way.

An alternative technique A shown in FIG. 4 is similar to technique Bexcept that the negative mold which is used to form the liner is alsoused as a liner support during filling. Thus, in Step 40 the polymericmaterial of the liner would be placed on the negative side of a mold ofthe desired shape, by a suitable coating process. The liner would thenbe filled with the metal powder without removing the liner from its moldin Step 41. Finally, the liner and its contents would be isopressed inthe normal way in Step 45. This technique can be particularly useful forlarge sized parts where the liner support may be necessary to maintainthe loose powder in shape before and during pressing.

Technique B shown in FIG. 5, is similar to technique C except that intechnique B the liner is so pliable that it must be supported duringfilling and handling or it will suffer significant deterioration of itsshape. Filling is accomplished while the liner is being supported.

As mentioned previously, the preferred method for removing the linerfrom the post-isopressed workpiece is technique C shown in FIG. 9. Inthis technique, after the workpiece and the liner has been isopressed inStep 51, they are moved to a preheating Step 52 which is normallynecessary in sintering operations and then to the sintering furnace inStep 53. The material from which the liner is made is chosen todecompose without adverse effect in the preheating zone of the sinteringfurnace. Preferably, the decomposition would result in an innocuous gas,but, since most sintering furnaces are provided with suitableventilating systems, the innocuousness of the products is not absolutelynecessary. This technique is particularly applicable to use inconjection with technique A and B where the liner is intentionally madeextremely thin and recycling of the liner material is not economic.

In an alternative liner removal technique X, shown in FIG. 7, after theisopressing Step 54, the liner is physically stripped off of the "green"workpiece in Step 55. The workpiece is then preheated in Step 56 andsintered in Step 57. This technique is particularly adapted for use withtechnique A.

Technique Y, shown in FIG. 8, involves the same steps as technique X,except that the stripping operation occurs after the preform has beenpreheated. This allows a thicker and perhaps less flexible liner to beremoved from the "green" workpiece without injuring the workpiece. Thus,the isopressing is accomplished in Step 58, the workpiece is preheatedin Step 59 and then the liner is stripped off in Step 60. Then theworkpiece is sintered in Step 61 in the normal manner. This procedure isparticularly adapted for technique C where the liner is less flexible.

It is obvious that minor changes may be made in the form andconstruction of the invention without departing from the material spiritthereof. It is not, however, desired to confine the invention to theexact form herein shown and described, but it is desired to include allsuch as properly come within the scope claimed.

The invention having been thus described, what is claimed as new anddesired to secure by Letters Patent is: .[.1. A process for forming aworkpiece of a predetermined shape from powdered metal, comprising thesteps of: contents and closed by a portion of the press beforepressing..]. .[.2. A process for forming a workpiece of a predeterminedshape from powdered metal, comprising the steps of:(a) forming asemi-rigid liner of heat-decomposible material having generally the saidshape, (b) adding a quantity of said powdered metal into the liner, (c)providing the liner with a liquid-impervious sealing element whichcompletely envelops it, (d) isostatically compacting the sealingelement, the liner and the contents, (e) heating the liner and contentsto cause decomposition of the liner and sintering the contents, and (f)forging the contents, wherein the liner is made of a foamed polymer inwhich the bubbles can collapse permanently during the compactingstep..]. .[.3. A process for forming a workpiece of a predeterminedshape from powdered metal, comprising the steps of: (a) forming asemi-rigid liner of heat-decomposible material having generally the saidshape, the liner being formed of a foamed polymer, (b) adding a quantityof said powdered metal into the liner, (c) providing the liner with aliquid-impervious sealing element which completely envelops it, (d)isostatically compacting the sealing element, the liner and thecontents, (e) heating the liner and contents to cause decomposition ofthe liner and sintering the contents, and (f) forging the contents,wherein the sealing element is a thin elastomer bag in which the lineris enclosed, which bag is removed before the liner and contents areheated..]. .Iadd.4. A process for forming a workpiece of a predeterminedshape from powdered metal, comprising the steps of:(a) forming asemi-rigid liner of heat-decomposible material having generally the saidshape, (b) adding a quantity of said powdered metal into the liner, (c)inserting the liner and contents into the open end of an elastomericliquid-impervious bag which forms part of an isostatic press providedwith means for sealing the open end of the bag, (d) sealing the open endof the bag, (e) isostatically compacting the bag against the liner sothat the bag completely envelops the liner to compact the powdered metalin the liner, (f) removing the liner and contents from the bag, (g)heating the liner and contents to cause decomposition of the liner andsintering the contents to form a rough workpiece, and (h) forging therough workpiece. .Iaddend. .Iadd.5. A process for forming a workpiece ofa predetermined shape from powdered metal, comprising the steps of:(a)forming a semi-rigid liner of heat-decomposible foamed polymer havinggenerally the said shape, (b) adding a quantity of said powdered metalinto the liner, (c) inserting the liner and contents into an elastomericliquid-impervious sealing element, (d) isostatically compacting the bagagainst the liner so that the sealing element completely envelops theliner to compact the powdered metal in the liner, (e) removing the linerand contents from the sealing element, (f) heating the liner andcontents to cause decomposition of the liner and sintering the contentsto form a rough workpiece, and (g) forging the rough workpiece..Iaddend. .Iadd.6. A process for forming a workpiece of a predeterminedshape from powdered metal, comprising the steps of: (a) forming asemi-rigid liner of heat-decomposible foamed polymer having generallythe said shape, (b) adding a quantity of said powdered metal into theliner, (c) inserting the liner and contents into an elastomericliquid-impervious bag, (d) isostatically compacting the bag against theliner so that the bag completely envelops the liner to compact thepowdered metal in the liner, (e) removing the liner and contents fromthe bag, (f) heating the liner and contents to cause decomposition ofthe liner and sintering the contents to form a rough workpiece, and (g)forging the rough workpiece. .Iaddend. .Iadd.7. A process for forming aworkpiece of a predetermined shape from powdered sinterable material formechanical finishing, comprising the steps of:(a) forming a semi-rigidliner of heat-decomposible material having generally the said shape, (b)adding a quantity of said powdered material into the liner, (c)inserting the liner and contents into the open end of an elastomericliquid-impervious bag which forms part of an isostatic press providedwith means for sealing the open end of the bag, (d) pressurizing thepress to force the bag against the liner so that the bag completelyenvelops the liner to compact the powdered material in the liner, (e)removing the liner and contents from the bag, and (f) heating the linerand contents to cause decomposition of the liner and sintering thecontents to form a rough workpiece. .Iaddend. .Iadd.8. A process forforming a rough workpiece of a predetermined shape from powderedsinterable material for mechanical finishing, comprising the stepsof:(a) forming a semi-rigid liner of heat-decomposible foamed polymerhaving generally the said shape, (b) adding a quantity of said powderedmaterial into the liner, (c) inserting the liner into an elastomericliquid-impervious sealing element, (d) isostatically pressurizing thesealing element and the contents so as to force the sealing elementagainst the liner so that the sealing element completely envelops theliner to permanently collapse the bubbles of the foamed polymer and tocompact the powdered material in the liner, (e) removing the liner andcontents from the sealing element, and (f) heating the liner andcontents to cause decomposition of the liner and sintering the contentsto form a rough workpiece. .Iaddend. .Iadd.9. A process for forming arough workpiece of a predetermined shape from powdered sinterablematerial for mechanically finishing, comprising the steps of:(a) forminga semi-rigid liner of heat-decomposible foamed polymer having generallythe said shape, (b) adding a quantity of said powdered material into theliner, (c) enclosing the liner within a liquid-impervious elastomer bag,(d) isostatically pressurizing the bag so that the bag completelyenvelops the liner and contents to force the bag against the liner andto compact the powdered material in the liner, (e) removing the bag andheating the liner and contents to cause decomposition of the liner andsintering the contents to form a rough workpiece. .Iaddend.